class Solution 
{
    int dx[4] = {0, 0, 1, -1};
    int dy[4] = {1, -1, 0, 0};

public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& mat) 
    {
        int m = mat.size(), n = mat[0].size();

        // dist[i][j] == -1表示：没有搜索过
        // dist[i][j] != -1表示：最短距离
        vector<vector<int>> dist(m, vector<int>(n, -1));
        queue<pair<int, int>> q;

        // 1. 把所有的源点加入到队列中
        for(int i = 0; i < m; i++)
            for(int j = 0; j < n; j++)
                if(mat[i][j] == 0)
                {
                    q.push({i, j});
                    dist[i][j] = 0;
                }
        
        // 2. 一层一层地往外扩
        while(q.size())
        {
            auto [a, b] = q.front();
            q.pop();
            for(int i = 0; i < 4; i++)
            {
                int x = a + dx[i], y = b + dy[i];
                if(x >= 0 && x < m && y >= 0 && y < n && dist[x][y] == -1)
                {
                    dist[x][y] = dist[a][b] + 1;    // 上一层的 dist 里面的最短距离 +1
                    q.push({x, y});
                }
            }
        }

        return dist;
    }
};